Heretofore, as Si.sub.3 N.sub.4 material, those described in, for example, Japanese Patent Unexamined Publication (KOKAI) Sho 49-21091, Sho 57-95873, Sho 59-182276, etc. have been known.
As production processes for them, there are a production method for .beta.-Si.sub.3 N.sub.4 in a Si.sub.3 N.sub.4 -Y.sub.2 O.sub.3 -Al.sub.2 O.sub.3 system by adding Y.sub.2 O.sub.3 and Al.sub.2 O.sub.3 or in a Si.sub.3 N.sub.4 -Y.sub.2 O.sub.3 -MgO-Al.sub.2 O.sub.3 system by adding Y.sub.2 O.sub.3 -MgO-Al.sub.2 O.sub.3 as a sintering aid, as well as a production method for .alpha.'-Si.sub.3 N.sub.4 and .beta.-Si.sub.3 N.sub.4 in a Si.sub.3 N.sub.4 -Y.sub.2 O.sub.3 -AlN series .alpha.-sialon or Si.sub.3 N.sub.4 -Y.sub.2 O.sub.3 -Al.sub.2 O.sub.3 -AlN series by adding Y.sub.2 O.sub.3 -AlN.
However, a glass phase comprising Si, N and additive ingredients is present between Si.sub.3 N.sub.4 particles, which bind crystal grains of Si.sub.3 N.sub.4 in any of the sintered bodies.
The glass phase is softened at high temperature to reduce the strength. That is, deterioration of the strength is remarkable at a temperature higher than 800.degree. C. and the strength is reduced to about one-half, in relation to normal temperature bending strength, at a temperature higher than 1200.degree. C.
For preventing the strength deterioration at high temperature, a Si.sub.3 N.sub.4 series material free from any glass phase, for example, a solid-solution-type Si.sub.3 N.sub.4 such as .beta.-sialon or a reaction-sintered Si.sub.3 N.sub.4 compound of Si.sub.3 N.sub.4 alone may be satisfactory. However, although these materials show less deterioration of strength at high temperature the strength itself is low at normal temperature and they are not suitable for structural material and tool material.
As for other means, although it is possible to prevent softening at high temperature by crystallizing the glass phase, this requires heat treatment in addition to sintering, making the steps complicated.
The principal object of the present invention is to obtain silicon nitride material having bending strength at normal temperature comparable with that of conventional silicon nitride having a glass phase but, in addition having much less deterioration of strength at high temperature.